System and method for controlling a system that includes fixed speed and variable speed compressors

ABSTRACT

A system and method for controlling a system that includes fixed speed and variable speed compressors are described. The method generally allows the system, for example, a heating, ventilating, and air condition (HVAC) system that includes fixed speed and variable speed compressors, to maximize unit modulating capability. The method allows the use of a variable speed compressor that is relatively smaller, which can lead to cost savings, easier installation, manufacturing, etc.

FIELD

The embodiments disclosed herein relate generally to a system and methodfor controlling a system that includes fixed speed and variable speedcompressors.

BACKGROUND

Control of a compressor utilized in a refrigeration circuit within arefrigeration system is known. Improvements in control of refrigerationsystems that include a compressor may be made.

SUMMARY

The embodiments described herein are directed to a system and method forcontrolling a system that includes a fixed speed compressor and avariable speed compressor. The method generally allows the system, forexample, a refrigeration or a heating, ventilating, and air condition(HVAC) system that includes fixed speed and variable speed compressors,to maximize unit modulating capability. The method allows the use of avariable speed compressor that has relatively smaller capacity, whichcan lead to cost savings, easier installation, manufacturing, etc.

In some embodiments, the system includes one variable speed compressor,at least one fixed speed compressor, a condenser and an evaporator. Insome examples, the system can include more than one variable speedcompressor. In some examples, the system can further include a controlunit that is configured to control the system by executing a controlprogram or algorithm that is stored in a memory of the control unit.

In some examples, the system includes first and second fixed speedcompressors, and the control unit is configured so that the fixed speedcompressors and the variable speed compressor operate in certainoperational stages.

The term “operational stage” means the operational state of each of thecompressors. The term “operating mode” refers to a capacity of thecompressors operating in a certain operational stage.

The operational state of the fixed speed compressor can be the on stateor the off state. The operational state of the variable speed compressorcan be the off state, variable speeds ranging from a minimum speed to amaximum speed. In some examples, the operational stages include thefollowing: Stage 0, Stage 1 min, Stage 1 max, Stage 2 min, Stage 2 max,Stage 3 min and Stage 3 max.

At Stage 0, the first and second fixed speed compressors and thevariable speed compressor operate in the off state so that the speeds ofthe fixed speed compressors and the variable speed compressor are at 0revolutions per second (rps).

At Stage 1 min, the speed of the variable speed compressor ramps up from0 rps until a minimum speed is reached. In some examples, the speed ofthe variable speed compressor ramps up at a constant rate and/or avariable rate. In some examples, the ramp rate can be different or thesame as that in the other operational stages. For example, the ramp ratein Stage 1 min can be the same as or different from that of Stage 1 max,Stage 2 max, and/or Stage 3 max. In some examples, the ramp rate ispredetermined. In some instances, the ramp rate is predetermined basedon the type of compressors utilized, e.g., manufacturer, size, etc. ofthe compressors. The first fixed speed compressor and the second fixedspeed compressor operate in the off state so that their speeds are at 0rps.

At Stage 1 max, the speed of the variable speed compressor can ramp upfrom the minimum speed to a maximum speed. In some examples, the speedof the variable speed compressor ramps up at a constant rate and/or avariable rate. The first fixed speed compressor and the second fixedspeed compressor operate in the off state so that the speeds are at 0rps.

At Stage 2 min, the first fixed speed compressor operates in the onstate and the second fixed speed compressor operates in the off state sothat the speed of the second fixed compressor is at 0 rps. The speed ofthe variable speed compressor is set at or ramped down to a minimumspeed. Note that the minimum speed of Stage 2 min can be different fromthe minimum speed of Stage 1 min.

At Stage 2 max, the first fixed speed compressor operates in the onstate and the second fixed speed compressor operates in the off state sothat the speed of the second fixed compressor is at 0 rps. The speed ofthe variable speed compressor ramps up from a minimum speed to a maximumspeed. In some examples, the speed of the variable speed compressorramps up at a constant rate and/or a variable rate. Note that themaximum speed of Stage 2 max can be different from the maximum speed ofStage 1 max.

At Stage 3 min, both the first fixed speed compressor and the secondfixed speed compressor operate in the on state. The speed of thevariable speed compressor is set at or ramped down to a minimum speed.Note that the minimum speed of Stage 3 min can be different from theminimum speed of Stage 1 min and/or the minimum speed of Stage 2 min.

At Stage 3 max, both the first fixed speed compressor and the secondfixed speed compressor operate in the on state. The speed of thevariable speed compressor ramps up from a minimum speed to a maximumspeed. In some examples, the speed of the variable speed compressorramps up at a constant rate and/or a variable rate. Note that themaximum speed of Stage 3 max can be different from the maximum speed ofStage 2 max and/or the maximum speed of Stage 2 max.

In some examples, the stages listed above, that is, Stage 0, Stage 1min, Stage 1 max, Stage 2 min, Stage 2 max, Stage 3 min and Stage 3 max,occur sequentially in the listed order when the load is increasing. Insome examples, the stages listed above occur in reverse order when theload is decreasing.

In some examples, the control unit implements an algorithm to controlthe operation of the compressors. The algorithm generally involvesmodulating the speed of a variable speed compressor relative to thefixed speed compressors based on a measured parameter and a set point ofthe measured parameter. In some examples, the control unit implements analgorithm to control the operation of the compressors using a PIcontroller.

In some instances, the algorithm can involve:

(a) measuring a parameter, for example, discharge air temperature (DAT);

(b) determining a PI capacity value based on the parameter measured in(a) and a set point using the PI controller;

(c) determining an operating mode based on the PI capacity valuedetermined in (b);

(d) operating the fixed speed and variable speed compressors based onthe determination made in (c);

(e) determining an operating mode after (d);

(f) determining the operational state of the fixed speed compressor;

(g) operating the fixed speed compressor based on the determination madein (f);

(h) determining a speed of the variable speed compressor using the PIcontroller; and

(i) operating the variable speed compressor based on the determinationmade in (h).

In some examples, in step (c), the PI capacity value is compared to oneor more preconfigured values to determine the operating mode, where theoperating mode indicates the applicable operational stage(s) based onthe determined PI capacity value. In some examples, the preconfiguredvalues are based on the configuration of the system, for example, thetype of compressors used, the number of compressors used etc.

In some instances, the determination made in steps (c) and (e) candepend on whether there is a capacity/stage gap for the variable speedcompressor relative to the fixed speed compressors.

In some examples, the ramp rate in the operational stage(s) is limitedusing the PI controller. In some examples, limiting the ramp rateinvolves determining a change in speed of the variable speed compressorusing the PI controller, comparing the determined change to apredetermined value, and based on the comparison, limiting the ramp rateof the variable speed compressor.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout.

FIG. 1 is a schematic illustration of a system for controlling theoperation of fixed speed and variable speed compressors, according toone embodiment.

FIGS. 2A-2C are flow charts of the processes involved in controlling thefixed speed and variable speed compressors, according to one embodiment.

FIGS. 3A and 3B illustrate the overall concept of the term“capacity/stage gap(s)”, according to one embodiment.

FIG. 4 shows a graph of the relation between the operating modes andoperational stages where there is no capacity/stage gap, i.e., where acapacity/stage overlap, according to one embodiment.

FIG. 5 shows a graph of the relation between the operating modes andoperational stages where there is a capacity/stage gap, according to oneembodiment.

DETAILED DESCRIPTION

The embodiments described herein are directed to a system and method forproviding control in a system that includes a variable speed compressorand at least one fixed speed compressor. In some examples, the systemcan include more than one variable speed compressor.

The system can be any system that utilizes a variable speed compressorand one or more fixed sped compressors, including, but not limited to,water source heat pumps, unitary systems, split systems, self-containedsystems, outdoor air units and airside, terminal devices and generallyany temperature control equipment that utilizes one or more variablespeed compressor and one or more fixed speed compressor. Airside andterminal devices include air handlers, make-up air gas heating systems,ventilation fans, blower coil air handlers, HVAC fan coil units,electric wall fins, unit ventilators and variable air volume units.

In some examples, the system can be a large tonnage unit. In someinstances, the large tonnage unit has an overall compressor capacitybetween about 6 to about 12 tons. In some other examples, the system canbe a larger tonnage unit. In some instances, the larger tonnage unit hasan overall compressor capacity between about 12.5 to about 162.0 tons.

FIG. 1 provides a schematic illustration of one embodiment of thedisclosed system (see system 100 in FIG. 1). The system 100 includes avariable speed compressor 105, a first fixed speed compressor 108 and asecond fixed speed compressor 112.

Note that FIG. 1 shows an example of a system including one variablespeed compressor 105 and two fixed speed compressors 108 and 112.However, the number of each of the compressors that can be included inthe system 100 can be any number that is suitable for use in arefrigeration and/or a HVAC system. Further details of a system thatincludes more than one variable speed compressor is provided at the endof the Detailed Description below.

The term “fixed speed compressor” means a compressor that operates at afixed speed. The first fixed speed compressor 108 and the second fixedspeed compressor 112 can be connected to motors 116 and 119,respectively, and can be operated as is generally known in the art. Thefirst fixed speed compressor 108 and the second fixed speed compressor112 can be controlled, for example, by a control unit 121 (note that thecontrol unit 121 will be discussed more in detail below) so that each ofthe first fixed speed compressor 108 and the second fixed speedcompressor 112 operates in the on state or off state. In the on state,each of the first fixed speed compressor 108 and the second fixed speedcompressor 112 operates at a fixed speed, e.g., a speed at anywherebetween 25 and 100 revolutions per second (rps). In some examples, oneor both of the motors 116 and 119 are induction motors. In someinstances, one or both of the fixed speed compressors 108 and 112 run ona 60 HZ power supply, and one or both of the fixed speed compressors 108and 112 operate at a fixed speed of about 60 rps. In the off state, eachof the first fixed speed compressor 108 and the second fixed speedcompressor 112 operates at 0 rps.

The term “variable speed compressor” means a compressor that operates atvariable speeds, as generally understood in the art. The variable speedcompressor 105 can be connected to a motor 124 that is driven by avariable speed drive 126, as is generally known in the art. The speed ofthe variable speed compressor 105 can be controlled, for example, by thecontrol unit 121, so that the variable speed compressor 105 operates atvariable speeds, for example, a range of speeds including a minimumspeed and a maximum speed. The minimum speed can be, for example, about25 rps, and the maximum speed can be, for example, 100 rps. Note thatthese speeds are provided as examples only. In some examples, theminimum speed and/or the maximum speed will depend on the configurationof the system 100 utilized, for instance, the type of variable speedcompressor used, the capacity of the variable speed compressor relativeto the fixed speed compressors, etc. When the variable speed compressor105 is turned off, the variable speed compressor 105 operates at 0 rps.

Each of the fixed speed compressors 108 and 112 and the variable speedcompressor 105 can be any compressor type that is suitable for use in arefrigeration and/or a HVAC system, and can include, but is not limitedto, reciprocating, scroll, rotary, screw, centrifugal, etc.

In some examples, each of the fixed speed compressors 108 and 112 andthe variable speed compressor 105 can be in fluid communication with acondenser 129 and a cooling coil 131. The cooling coil 131, thecondenser 129 and the compressors 105, 108, and 112 can utilize arefrigeration loop that is generally known in the art. In some instancesof the refrigeration loop, the compressors 105, 108, and 112 can feedhigh-pressure and high-temperature refrigerant gas to the condenser 129.The refrigerant vapor that is delivered to the condenser 129 then canenter into a heat exchange relationship with a fluid, for example, air.In some embodiments, the condensed liquid refrigerant from the condenser129 then can flow through an expansion device (not shown) to anevaporator (not shown). In some instances, when an evaporator is used, asecondary liquid, e.g., water, that has flowed into the evaporator thencan enter into a heat exchange relationship with the low pressure/lowtemperature liquid refrigerant to chill the temperature of the secondaryliquid. The chilled secondary liquid can then run through the coolingcoil 131, and the refrigerant liquid in the evaporator can undergo aphase change to a refrigerant vapor as a result of the heat exchangerelationship with the secondary liquid. It will be appreciated that ifan evaporator is not employed, the cooling coil 131 may act as theevaporator in the system 100. The refrigerant vapor then can return tothe compressors 105, 108, and 112 to complete the refrigeration loop.

The system 100 also can include a sensor 135 for measuring a parameter.In some examples, the parameter can be a discharge air temperature (DAT)and/or a space temperature.

The control unit 121 of the system 100 generally can include a processor(not shown), a memory (not shown), a clock (not shown), an input/output(I/O) interface (not shown) and a PI controller (not shown) and can beconfigured to receive data as input from various components within thesystem 100, and send command signals as output to various componentswithin the system 100.

In some examples, during operation, the control unit 121 can receiveinformation, for instance, from the first fixed speed compressor 108,the second fixed speed compressor 112, the variable speed compressor105, and/or the sensor 135 through the I/O interface, process thereceived information using the processor based on an algorithm stored inthe memory, and then send command signals, for instance, to thecomponents involved in the refrigeration loop including the first fixedspeed compressor 108, the second fixed speed compressor 112, and/or thevariable speed compressor 105.

For example, the control unit 121 can receive information regarding theDAT from the sensor 135, process the data, and then based on the data,send a command signal to the variable speed compressor 105 so as tocontrol the speed of the variable speed compressor 105 and/or send acommand signal to the first fixed speed compressor 108 and/or the secondfixed speed compressor 112 to control the operation of the respectivecompressors 108 and 112. It is to be realized that the control unit 121can be configured to receive information and send command signals toother components that are generally known to be included in a systemthat utilizes fixed speed and variable speed compressors.

Details of the various algorithms that can be stored in the memory willnow be provided below.

Generally, the system 100 is configured so that command signals are sentfrom the control unit 121 to the fixed speed compressors 108 and 112 andthe variable speed compressor 105, and after receiving the commandsignals, the respective compressors 105, 108 and 112 operate, forexample, in the following operational stages: Stage 0, Stage 1 min,Stage 1 max, Stage 2 min, Stage 2 max, Stage 3 min and Stage 3 max.Details of each of the stages are provided below.

At Stage 0, each of the first fixed speed compressor 108, the secondfixed speed compressor 112 and the variable speed compressor 105operates in the off state so that the speed of each of the fixed speedcompressors 108 and 112 and the variable speed compressor 105 is at 0rps.

At Stage 1 mil, the speed of the variable speed compressor 105 ramps upfrom 0 rps until a minimum speed is reached. In some examples, the speedof the variable speed compressor 105 ramps up at a constant rate and/ora variable rate. In one implementation, the constant rate and/or thevariable speed is predetermined. In some examples, the constant rate isan increase of about 2 rps. In some examples, the variable rate is arate that varies between 2 and 4 rps. In other examples, the constantrate and/or the variable rate can be determined based on theconfiguration of the system 100. In some examples, a PI controller canbe used to limit the ramp rate as will be discussed below. The firstfixed speed compressor 108 and the second fixed speed compressor 112operate in the off state so that the speeds are at 0 rps.

At Stage 1 max, the speed of the variable speed compressor 105 can rampup from a minimum speed to a maximum speed. In some examples, the speedof the variable speed compressor 105 ramps up at a constant rate and/ora variable rate. In one implementation, the constant rate and/or thevariable speed is predetermined. In some examples, the constant rate isan increase of about 2 rps. In some examples, the variable rate is arate that varies between 2 and 4 rps. In other examples, the constantrate and/or the variable rate can be determined based on theconfiguration of the system 100. In some examples, a PI controller canbe used to limit the ramp rate as described below. The first fixed speedcompressor 108 and the second fixed speed compressor 112 operate in theoff state so that the speeds are at 0 rps.

At Stage 2 min, the first fixed speed compressor 108 operates in the onstate and the second fixed speed compressor 112 operates in the offstate so that the speed of the second fixed compressor is at 0 rps. Thespeed of the variable speed compressor 105 is set at a minimum speed.

At Stage 2 max, the first fixed speed compressor 108 operates in the onstate and the second fixed speed compressor 112 operates in the offstate so that the speed of the second fixed compressor is at 0 rps. Thespeed of the variable speed compressor 105 ramps up from a minimum speedto a maximum speed. In some examples, the speed of the variable speedcompressor 105 ramps up at a constant rate and/or a variable rate. Inone implementation, the constant rate and/or the variable speed ispredetermined. In some examples, the constant rate is an increase ofabout 2 rps. In some examples, the variable rate is a rate that variesbetween 2 and 4 rps. In other examples, the constant rate and/or thevariable rate can be determined based on the configuration of the system100. In some examples, a PI controller can be used to limit the ramprate as will be discussed below.

At Stage 3 min, both the first fixed speed compressors 108 and thesecond fixed speed compressor 112 operate in the on state. The speed ofthe variable speed compressor 105 is set at a minimum speed.

At Stage 3 max, both the first fixed speed compressor 108 and the secondfixed speed compressor 112 operate in the on state. The speed of thevariable speed compressor 105 ramps up from a minimum speed to a maximumspeed. In some examples, the speed of the variable speed compressor 105ramps up at a constant rate and/or a variable rate. In oneimplementation, the constant rate and/or the variable speed ispredetermined. In some examples, the constant rate is an increase ofabout 2 rps. In some examples, the variable rate is a rate that variesbetween 2 and 4 rps. In other examples, the constant rate and/or thevariable rate can be determined based on the configuration of the system100. In some examples, a PI controller can be used to limit the ramprate as will be discussed below. In some instances, Stage 3 max is thefull capacity for the exemplary set of compressors 105, 108 and 112.

In some examples where the PI controller is used to limit the ramp rate,limiting the ramp rate involves determining a change in speed of thevariable speed compressor using the PI controller, comparing thedetermined change to a predetermined value, and based on the comparison,limiting the ramp rate of the variable speed compressor. Thepredetermined value can be, e.g., 2 rps. In some example, if thedetermined change is greater than a predetermined value where the loadis increasing, the ramp rate is limited to the predetermined value.Otherwise, the ramp rate may not be limited. In some examples, if thedetermined change is less than a predetermined value where the load isdecreasing, the ramp rate is limited to the predetermined value.Otherwise, the ramp rate may not be limited.

In some examples, the minimum speed and/or the maximum speed of thevariable speed compressor 105 for each of the respective stages can bedifferent from or the same as one another. For instance, the minimumspeed of the variable speed compressor 105 in Stage 1 min can bedifferent from or the same as that of Stage 2 min and/or Stage 3 min.Likewise, the maximum speed of the variable speed compressor 105 inStage 1 max can be different from or same as that of Stage 2 max and/orStage 3 max. In some instances, the minimum speed and maximum speedselection is a tradeoff among, for example, factors including cost,energy efficiency and acoustics. In some other examples, the minimumspeed and/or maximum speed is (are) predetermined for the selectedvariable speed compressor 105. For instance, the minimum speed and/ormaximum speed is (are) predetermined based on the size and model of thevariable speed compressor 105 utilized. In some examples, the algorithmdescribed below utilizes the minimum speed and/or maximum speed tonormalize dynamic behavior of the system 100.

In some examples, the operational stages listed above, that is, Stage 0,Stage 1 min, Stage 1 max, Stage 2 min, Stage 2 max, Stage 3 min andStage 3 max, can occur sequentially in the listed order when the load isincreasing. Note that the compressors are not required to operate in allof the operational stages. That is, the system 100 is capable ofmodulating between the operational stages so that the compressorsoperate in only a subset of the listed operational stages. The subset ofthe operational stages in which the compressors 105, 108 and 112 operatecan be determined by an algorithm, for example the algorithm that isdiscussed in detail below.

In some examples, the operational stages listed above can occur inreverse order when the load is decreasing, for example, in the followingsequential order: Stage 3 max, Stage 3 min, Stage 2 max, Stage 2 min,Stage 1 max, Stage 1 min and Stage 0.

Generally, the control unit 121 can be configured to implement thedisclosed method of controlling the system 100 as illustrated in FIGS.2A-2C. In general, the process described in FIGS. 2A-2C is executed bythe processor executing program instructions (algorithm(s)) stored inthe memory of the control unit 121.

With reference to FIG. 2A, in one embodiment, the disclosed method oralgorithm 200 initiates at step 206 and proceeds to step 209 where adetermination is made as to a parameter using the sensor 135. In someexamples, the parameter can be space temperature. In some otherexamples, the parameter can be discharge air temperature (DAT). In thedescription that follows, the algorithm 200 will be described using DATas the parameter. However, it is to be realized that the spacetemperature can replace the DAT in the description that follows.

After step 209, the algorithm 200 proceeds to 215 where a determinationis made as to a PI capacity value using the PI controller. In someexamples, the PI capacity value is determined based on the DAT measuredin step 209 and a set point of the DAT. In some examples, the PIcapacity value can be determined, for example, by applying a gain valueto the difference between the measured DAT and the set point. In someexamples, the gain value can be adjusted, for example, by consideringthe supply air flow across an evaporator coil. In some instances, the PIcontroller has a deadband of about 0.5 to about 1° F. so that when theDAT is about 0.5 to about 1° F. above the set point, the algorithm 200continues, and when the DAT is about 0.5 to about 1° F. below the setpoint, the algorithm 200 ends. In some instances, a wider deadband canbe used at Stage 0 to Stage 1 min to minimize cycling. Note that cyclingrefers to the cycling step 235 in FIG. 2C, which will be discussed infurther detail below.

After step 215, a determination is made as to an operating mode based onthe PI capacity value at 221. The operating mode is a parameter that isused by the algorithm 200 to indicate the applicable operationalstage(s) based on the determined PI capacity value. In some examples,the operating mode is determined by comparing the PI capacity value withpreconfigured values, for example, using a lookup table. In someexamples, the operating mode is determined by normalizing the PIcapacity value based on the configuration of the system 100. In someexamples, the preconfigured values are based on the configuration of thesystem, for example, the type of compressors used, the number ofcompressors used, etc.

In some examples, the type of parameters used for the operating modedepends on whether there is(are) capacity/stage gap(s) for the variablespeed compressor 105 relative to the fixed speed compressors 108 and112.

The meaning of “capacity/stage gap(s)” will now be described withreference to FIGS. 3A and 3B. FIGS. 3A and 3B illustrate the overallconcept of the term “capacity/stage gap(s)”. The term “capacity/stagegap(s)” generally refers to the gap in capacity of the variable speedcompressor 105 relative to the fixed speed compressors 108 and 112. Theterm “capacity” means the tons the compressor will produce based oncertain operating conditions.

Generally, the “capacity/stage gap(s)” concept illustrates how thealgorithm 200 modulates the operation of the fixed speed compressors 108and 112 and the variable speed compressor 105 based on the configurationof the system 100. The modulation of the operation of the fixed speedcompressors 108 and 112 and the variable speed compressor 105 can bedone by adjusting the speed of the variable speed compressor 105 and/orby a cycling operation to meet a set point of the PI controller.

Each of a first stage capacity box 302, a second stage capacity box 308and a third stage capacity box 310 in FIGS. 3A and 3B indicates acapacity range of the variable speed compressor 105 under differentoperating states of the variable speed compressor 105 and the firstfixed speed compressor 108 and the second fixed speed compressor 112. Inparticular, the first stage capacity box 302 indicates a capacity range305 of the variable speed compressor 105 when both the first fixed speedcompressor 108 and the second fixed speed compressor 112 are turned off.The second stage capacity box 308 indicates a capacity range 309 of thevariable speed compressor 105 when the variable speed compressor 105 isoperating between a minimum speed and a maximum speed, and the firstfixed speed compressor 108 is turned on and the second fixed speedcompressor 112 is turned off. The third stage capacity box 310 indicatesa capacity range 312 of the variable speed compressor 105 when thevariable speed compressor 105 is operating between a minimum speed and amaximum speed, and both the first fixed speed compressor 108 and thesecond fixed speed compressor 112 are turned on.

Referring to FIG. 3A, in some examples, there is no capacity/stage gapwhen a maximum capacity of the capacity range in one stage overlaps aminimum capacity of the capacity range in the subsequent stage. In someexamples, an overlapping region of the maximum capacity of the capacityrange in one stage and the minimum capacity of the capacity range in thesubsequent stage is about 2 to about 3%. In some examples, there is nocapacity/stage gap if a maximum capacity 302 max of the capacity range305 in the first stage capacity box 302 overlaps a minimum capacity 308min of the capacity range 309 in the second stage capacity box 308and/or if a maximum capacity 308 max of the capacity range 309 in thesecond stage capacity box 308 overlaps a minimum capacity 310 min of thecapacity range 312 in the third stage capacity box 310 as shown in FIG.3A.

Referring to FIG. 3B, on the other hand, a capacity/stage gap is presentwhen a maximum capacity of the capacity range in one stage does notoverlap a minimum capacity of the capacity range in the subsequentstage. In particular, there is a capacity/stage gap 315 if the maximumcapacity 302 max of the capacity range 305 in the first stage capacitybox 302 does not overlap the minimum capacity 308 min of the capacityrange 309 in the second stage capacity box 308 and/or if the maximumcapacity 308 max of the capacity range 309 in the second stage capacitybox 308 does not overlap the minimum capacity 310 min of the capacityrange 312 in the third stage capacity box 310 as shown in FIG. 3B.

Generally, the presence or absence of the capacity/stage gap depends onthe configuration of the system 100, for instance, the type of variablespeed compressor 105 utilized. For example, a capacity/stage gap mayoccur in systems where the variable speed compressor 105 has a smallcapacity relative to the fixed speed compressors 108 and 112. Ingeneral, in the instance where the capacity/stage gap is present, thealgorithm 200 will modulate the operation of the fixed speed compressors108 and 112 and the variable speed compressor 105 by using a cyclingoperation, whereas in the instance where the capacity/stage gap isabsent, the algorithm 200 will not use a cycling operation.

Details of Algorithm where there is No Capacity/Stage Gap

Referring back to FIG. 2A, in some examples, if there is nocapacity/stage gap, that is, where a capacity/stage overlap is present,then the parameters used for the operating mode in the algorithm 200include Stage 1 min capacity, Stage 1 max capacity and Stage 2 maxcapacity.

FIG. 4 shows a graph illustrating the overall relation between theoperating modes on the x-axis, namely, Stage 1 min capacity 402, Stage 1max capacity 406 and Stage 2 max capacity 408, and the operationalstages on the y-axis, namely, Stage 0 411, Stage 1 min 415, Stage 1 max418, Stage 2 min 421, Stage 2 max 425, Stage 3 min 427 and Stage 3 max431.

Referring to FIG. 4, Stage 1 min capacity 402 on the x-axis referencesthe unit capacity when the variable speed compressor 105 is operating atStage 1 min 415 on the y-axis, where both the first fixed speedcompressor 108 and the second fixed speed compressor 112 are turned off;Stage 1 max capacity 406 on the x-axis references the unit capacity whenthe variable speed compressor 105 is operating at Stage 1 max 418 on they-axis, where both the first fixed speed compressor 108 and the secondfixed speed compressor 112 are turned off; and Stage 2 max capacity 408on the x-axis references the unit capacity when the variable speedcompressor 105 is operating at Stage 2 max 425 on the y-axis, where thefirst fixed speed compressor 108 is turned on and the second fixed speedcompressor 112 is turned off.

Referring to FIG. 2A and FIG. 2B, after the operating mode has beendetermined at 221 as shown in FIG. 2A, the algorithm proceeds to thesteps in {circle around (1)}. Details of {circle around (1)} areprovided in FIG. 2B.

The steps in {circle around (1)} now will be described with reference toFIG. 2B. At 222, the variable speed compressor 105, the first fixedspeed compressor 108 and/or the second fixed speed compressor 112operate at the referenced operational stage based on the operating modedetermined in 221.

Then, at 223, a determination is made as to the DAT, and at 224, adetermination is made as to a PI capacity value using the PI controller.In some examples, the PI capacity value is determined based on the DATmeasured in step 223 and the set point of the DAT.

The algorithm 200 then proceeds to 225, where a determination is made asto an operating mode based on the PI capacity value determined at 224.In some instances, the parameters used for the operating mode canfurther include Stage 1 max cap+5 and Stage 2 max cap+5. In someexamples, Stage 1 max cap+5 and Stage 2 max cap+5 are used as dynamicbuffers to prevent cycling between the referenced operating stage and aprevious operating stage. Note that “+5” indicates a 5% differential inthe PI capacity values for the respective operating modes, andreferences a dynamic buffer value. Note also that “+5” is only anexample, and can be other values, for example, 0 to 20.

The meaning of “dynamic buffer” will be described with reference to FIG.4. As illustrated in FIG. 4, Stage 1 max capacity+5 446 on the x-axisreferences the unit capacity when the variable speed compressor 105 isoperating at Stage 2 min 421 on the y-axis, where the first fixed speedcompressor 108 is turned on and the second fixed speed compressor 112 isturned off, and Stage 2 max capacity+5 452 on the x-axis references theunit capacity when the variable speed compressor 105 is operating atStage 3 min 427 on the y-axis, where the first fixed speed compressor108 is turned on and the second fixed speed compressor 112 is turned on.

In the instance where the PI capacity value determined in 224 is withina dynamic buffer value and the operating mode is determined to be, forexample, Stage 1 max cap+5 446 on the x-axis, cycling does not occurbetween Stage 2 min 421 and Stage 1 max 418 in the region 438 on they-axis. If, for example, the operating mode is determined to be Stage 2max cap+5 452, cycling does not occur between Stage 3 min 427 and Stage2 max 425 in the region 441 on the y-axis. If the operating mode isdetermined to be Stage 1 min capacity 402, cycling does not occurbetween Stage 1 min 415 and Stage 0 411 in the region 435 on the y-axis.

Referring back to FIG. 2B, after step 225, a determination is made as tothe operational state of the first fixed speed compressor 108 and/or thesecond fixed speed compressor 112 at 226. A determination is also madeas to the speed of the variable speed compressor 105 based on the PIcapacity value at 227. Then, at 228, the first fixed speed compressor108 and/or the second fixed speed compressor 112 is operated based onthe determination made in 226, and at 229, the variable speed compressor105 is operated based on the determination made in 227.

Details of Algorithm in the Presence of Capacity/Stage Gap

In some examples, if a capacity/stage gap is present, then theparameters used for the operating mode in the algorithm 200 includeStage 1 min capacity. Stage 1 max capacity, Stage 2 min capacity, Stage2 max capacity and Stage 3 min capacity. FIG. 5 shows a graphillustrating the overall relation between the operating modes, namely,Stage 1 min capacity 502, Stage 1 max capacity 506, Stage 2 min capacity509, Stage 2 max capacity 513 and Stage 3 min capacity 516, and theoperational stages, namely, Stage 0 521, Stage 1 min 524, Stage 1 max531, Stage 2 min 536, Stage 2 max 539, Stage 3 min 541 and Stage 3 max545. The operating modes are provided on the x-axis, while theoperational stages are provided on the y-axis.

Referring to FIG. 5, Stage 1 min capacity 502 references the unitcapacity when the variable speed compressor 105 is operating at Stage 1min 524, where both the first fixed speed compressor 108 and the secondfixed speed compressor 112 are turned off; Stage 1 max capacity 506references the unit capacity when the variable speed compressor 105 isoperating at Stage 1 max 531, where both the first fixed speedcompressor 108 and the second fixed speed compressor 112 are turned off;Stage 2 min capacity 509 references the unit capacity when the variablespeed compressor 105 is operating at Stage 2 min 536 where the firstfixed speed compressor 108 is turned on and second fixed speedcompressor 112 is turned off; Stage 2 max capacity 513 references theunit capacity when the variable speed compressor 105 is operating atStage 2 max 539, where the first fixed speed compressor 108 is turned onand the second fixed speed compressor 112 is turned off; and Stage 3 mincapacity 516 references the unit capacity when the variable speedcompressor 105 is at Stage 3 min 541 and both the first fixed speedcompressor 108 and the second fixed speed compressor 112 are turned on.

Referring to FIGS. 2A and 2C, after the operating mode has beendetermined at 221, the algorithm proceeds to the steps in {circle around(1)}. Details of {circle around (1)} are provided in FIG. 2C.

The steps in {circle around (1)} now will be described with reference toFIG. 2C. At 232, the variable speed compressor 105, the first fixedspeed compressor 108 and/or the second fixed speed compressor 112operate at the referenced operational stage based on the operating modedetermined in 221.

Then, the algorithm proceeds to 235 where the variable speed compressor105, the first fixed speed compressor 108 and/or the second fixed speedcompressor 112 cycle between the referenced operational stage and asubsequent or previous operational stage.

Generally, the reason cycling occurs in the presence of capacity/stagegap is explained as follows with reference to FIG. 5. During operation,the system 100 can have too much cooling if running at, for example,Stage 2 min 536. In this instance, the DAT will become lower than theset point of the DAT, and as a result, the PI capacity value willdecrease. When PI capacity value becomes less than, for example, Stage 1max 531, cycling will occur between Stage 1 max 531 and Stage 2 min 536in the region 561 on the y-axis.

Further details of the cycling operation will be described withreference to FIG. 5. Cycling can occur between Stage 0 521 and Stage 1min 524 so that the variable speed compressor 105 cycles between the offstate and a minimum speed as shown by region 555 on the y-axis. Cyclingalso can occur between Stage 1 max 531 and Stage 2 min 536 so that thevariable speed compressor 105 cycles between a minimum and a maximumspeed and the first fixed speed compressor 108 cycles between the on andoff states as shown by region 561 on the y-axis. Cycling also can occurbetween Stage 2 max 539 and Stage 3 min 541 so that the variable speedcompressor 105 cycles between a minimum and a maximum speed and thefirst fixed speed compressor 108 cycles between the on and off state asshown by region 569 in the y-axis.

In some examples, the variable speed compressor 105, the first fixedspeed compressor 108 and/or the second fixed speed compressor 112 cyclea predetermined number of times, for example, 1-5 times. In otherexamples, the number of the times the variable speed compressor 105, thefirst fixed speed compressor 108 and/or the second fixed speedcompressor 112 cycle between the respective operational stages candepend on the PI capacity value.

Referring back to FIG. 2C, after step 235, the algorithm 200 proceeds to237 where a determination is made as to the DAT. Then, at 240, adetermination is made as to the PI capacity value based on the DATdetermined in 237 and the set point of the DAT. Then, at 242, adetermination is made as to the operating mode.

Referring to FIG. 2C, after step 242, a determination is made as to theoperational state of the first fixed speed compressor 108 and/or thesecond fixed speed compressor 112 at 248. A determination is also madeas to the speed of the variable speed compressor 105 based on the PIcapacity value at 259. Then, at 252, the first fixed speed compressor108 and/or the second fixed speed compressor 112 is operated based onthe determination made in 248, and at 261, the variable speed compressor105 is operated based on the determination made in 259.

In the above-described examples, a system including two fixed speedcompressors and one variable speed compressor is described. However, insome examples, the system 100 can include more than one variable speedcompressor. In one implementation, one of the fixed speed compressors108, 112 can be replaced with a variable speed compressor so that thesystem includes two variable speed compressors and one fixed speedcompressor. The algorithm utilized in a system including two variablespeed compressors and one fixed speed compressor would be the same asthe algorithm 200 discussed above, except that one of the fixed speedcompressors 108, 112 would be replaced with a variable speed compressor,and the variable speed compressor replacing one of the fixed speedcompressors 108, 112 would switch between speeds rather than turning onand off.

With regard to the foregoing description, it is to be understood thatchanges may be made in detail, especially in matters of the constructionmaterials employed and the shape, size and arrangement of the partswithout departing from the scope of the present invention. It isintended that the specification and depicted embodiment to be consideredexemplary only, with a true scope and spirit of the invention beingindicated by the broad meaning of the claims.

What is claimed is:
 1. A system for controlling the operation of avariable speed compressor and a fixed speed compressor, wherein thesystem comprises a fixed speed compressor; a variable speed compressor,and a control unit that includes a PI controller and is configured to:(a) determine a first parameter value; (b) determine a first PI capacityvalue based on the first parameter value determined in (a) and a setpoint using the PI controller; (c) determine a first operating modebased on the first PI capacity value determined in (b); (d) operate thefixed speed and variable speed compressors based on the determinationmade in (c); (e) determine a second operating mode after (d); (f)determine an operational state of the fixed speed compressor; (g)operate the fixed speed compressor based on the determination made in(f); (h) determine a speed of the variable speed compressor using the PIcontroller; and (i) operate the variable speed compressor according toan operational state based on the determination made in (h).
 2. Thesystem of claim 1, wherein the fixed speed compressor includes aplurality of fixed speed compressors, and the plurality of fixed speedcompressors includes a first fixed speed compressor and a second fixedspeed compressor.
 3. The system of claim 2, wherein the variable speedcompressor and the first and second fixed speed compressors have one ormore operational stages: Stage 0, Stage 1 min, Stage 1 max, Stage 2 min,Stage 2 max, Stage 3 min and Stage 3 max, wherein each of theoperational stages are different from one another, the one or moreoperational stages corresponds to the operational state.
 4. The systemof claim 3, wherein at the Stage 0, the first and second fixed speedcompressors and the variable speed compressor operate in the off stateso that the speeds of the fixed speed compressors and the variable speedcompressor are at 0 revolutions per second (rps); wherein at the Stage 1min, the speed of the variable speed compressor ramps up from 0 rpsuntil a minimum speed is reached, and the first and second fixed speedcompressors operate in the off state so that the speeds are at 0 rps;wherein at the Stage 1 max, the speed of the variable speed compressorramps up from a minimum speed to a maximum speed, and the first andsecond fixed speed compressors operate in the off state so that thespeeds are at 0 rps; wherein at the Stage 2 min, the first fixed speedcompressor operates in the on state and the second fixed speedcompressor operates in the off state so that the speed of the secondfixed compressor is at 0 rps, and the speed of the variable speedcompressor is set at a minimum speed; wherein at the Stage 2 max, thefirst fixed speed compressor operates in the on state and the secondfixed speed compressor operates in the off state so that the speed ofthe second fixed compressor is at 0 rps, and the speed of the variablespeed compressor ramps up from a minimum speed to a maximum speed;wherein at the Stage 3 min, both the first fixed speed compressor andthe second fixed speed compressor operate in the on state, and the speedof the variable speed compressor is set at a minimum speed; and whereinat the Stage 3 max, both the first fixed speed compressors and thesecond fixed speed compressor operate in the on state, and the speed ofthe variable speed compressor ramps up from a minimum speed to a maximumspeed.
 5. The system of claim 4, wherein a capacity/stage overlap of thevariable speed compressor relative to the fixed speed compressor ispresent, and the operating mode includes one or more of Stage 0, Stage 1min capacity, Stage 1 max capacity, Stage 1 max capacity+X, Stage 2 maxcapacity, Stage 2 max capacity+X and Stage 3 max capacity, wherein X isa dynamic buffer value expressed as a percentage.
 6. The system of claim5, wherein the Stage 0 capacity references a unit capacity when thefirst and second fixed speed compressors and the variable speedcompressor operate in the off state; the Stage 1 min capacity referencesthe unit capacity when the variable speed compressor is operating atStage 1 min, where both the first fixed speed compressor and the secondfixed speed compressor are turned off; wherein the Stage 1 max capacityreferences the unit capacity when the variable speed compressor isoperating at Stage 1 max, where both the first fixed speed compressorand the second fixed speed compressor are turned off; wherein the Stage2 max capacity references the unit capacity when the variable speedcompressor is operating at Stage 2 max, where the first fixed speedcompressor is turned on and the second fixed speed compressor is turnedoff; and wherein the Stage 3 max capacity references the unit capacitywhen both the first fixed speed compressor and the second fixed speedcompressor operate in the on state.
 7. The system of claim 5, wherein Xis between about 0 to about 20%.
 8. The system of claim 2, wherein acapacity/stage gap of the variable speed compressor relative to thefixed speed compressor is present, and the operating mode includes Stage0, Stage 1 min capacity, Stage 1 max capacity, Stage 2 min capacity,Stage 2 max capacity, Stage 3 min capacity and Stage 3 max capacity. 9.The system of claim 8, wherein the Stage 0 capacity references a unitcapacity when the first and second fixed speed compressors and thevariable speed compressor operate in the off state; wherein the Stage 1min capacity references the unit capacity when the variable speedcompressor is operating at Stage 1 min, where both the first fixed speedcompressor and the second fixed speed compressor are turned off; whereinthe Stage 1 max capacity references the unit capacity when the variablespeed compressor is operating at Stage 1 max, where both the first fixedspeed compressor and the second fixed speed compressor are turned off;wherein the Stage 2 min capacity references the unit capacity when thevariable speed compressor is operating at Stage 2 min where the firstfixed speed compressor is turned on; wherein the Stage 2 max capacityreferences the unit capacity when the variable speed compressor isoperating at Stage 2 max, where the first fixed speed compressor isturned on and the second fixed speed compressor is turned off; whereinthe Stage 3 min capacity references the unit capacity when the variablespeed compressor is at Stage 3 min and both the first fixed speedcompressor and the second fixed speed compressor are turned on; andwherein the Stage 3 max capacity references the unit capacity when boththe first fixed speed compressor and the second fixed speed compressoroperate in the on state.
 10. The system of claim 1, wherein in (c), thefirst PI capacity value is compared to one or more predetermined valuesto determine the applicable operational stage.
 11. A method ofcontrolling the operation of a variable speed compressor and a fixedspeed compressor in a refrigeration or a heating, ventilating and airconditioning system, the method comprising: (a) determining a dischargeair temperature (DAT); (b) determining a PI capacity value based on theDAT determined in (a) and a set point using a PI controller; (c)determining a first operating mode based on the PI capacity valuedetermined in (b); (d) operating the fixed speed compressors and thevariable speed compressor based on the determination made in (c); (e)determining a second operating mode after (d); (f) determining theoperational state of the fixed speed compressor; (g) operating the fixedspeed compressor based on the determination made in (f); (h) determininga speed of the variable speed compressor using the PI controller; and(i) operating the variable speed compressor according to an operationalstate based on the determination made in (h).
 12. The method of claim11, wherein the system includes a plurality of fixed speed compressorsand the plurality of fixed speed compressors includes first and secondfixed speed compressors, and the variable speed compressor and the firstand second fixed speed compressors have one or more operational stages:Stage 0, Stage 1 min, Stage 1 max, Stage 2 min, Stage 2 max, Stage 3 minand Stage 3 max, wherein each of the operational stages are differentfrom one another, the one or more operational stages corresponds to theoperational state.
 13. The method of claim 12, wherein at the Stage 0,the first and second fixed speed compressors and the variable speedcompressor operate in the off state so that the speeds of the fixedspeed compressors and the variable speed compressor are at 0 revolutionsper second (rps); wherein at the Stage 1 min, the speed of the variablespeed compressor ramps up from 0 rps until a minimum speed is reached,and the first and second fixed speed compressors operate in the offstate so that the speeds are at 0 rps; wherein at the Stage 1 max, thespeed of the variable speed compressor ramps up from a minimum speed toa maximum speed, and the first and second fixed speed compressorsoperate in the off state so that the speeds are at 0 rps; wherein at theStage 2 min, the first fixed speed compressor operates in the on stateand the second fixed speed compressor operates in the off state so thatthe speed of the second fixed compressor is at 0 rps, and the speed ofthe variable speed compressor is set at a minimum speed; wherein at theStage 2 max, the first fixed speed compressor operates in the on stateand the second fixed speed compressor operates in the off state so thatthe speed of the second fixed compressor is at 0 rps, and the speed ofthe variable speed compressor ramps up from a minimum speed to a maximumspeed; wherein at the Stage 3 min, both the first fixed speed compressorand the second fixed speed compressor operate in the on state, and thespeed of the variable speed compressor is set at a minimum speed; andwherein at the Stage 3 max, both the first fixed speed compressors andthe second fixed speed compressor operate in the on state, and the speedof the variable speed compressor ramps up from a minimum speed to amaximum speed.
 14. The method of claim 13, wherein a capacity/stageoverlap of the variable speed compressor relative to the fixed speedcompressor is present, and the operating mode includes one or more ofStage 0, Stage 1 min capacity, Stage 1 max capacity+X, Stage 2 maxcapacity+X and Stage 3 max capacity, wherein X is a dynamic buffer valueexpressed as a percentage.
 15. The method of claim 14, wherein the Stage0 capacity references a unit capacity when the first and second fixedspeed compressors and the variable speed compressor operate in the offstate; wherein the Stage 1 min capacity references the unit capacitywhen the variable speed compressor is operating at Stage 1 min, whereboth the first fixed speed compressor and the second fixed speedcompressor are turned off; wherein the Stage 1 max capacity referencesthe unit capacity when the variable speed compressor is operating atStage 1 max, where both the first fixed speed compressor and the secondfixed speed compressor are turned off; wherein the Stage 2 max capacityreferences the unit capacity when the variable speed compressor isoperating at Stage 2 max, where the first fixed speed compressor isturned on and the second fixed speed compressor is turned off; andwherein the Stage 3 max capacity references the unit capacity when boththe first fixed speed compressor and the second fixed speed compressoroperate in the on state.
 16. The method of claim 15, wherein acapacity/stage gap of the variable speed compressor relative to thefixed speed compressor is present, and the operating mode includes oneor more of Stage 0, Stage 1 min capacity, Stage 1 max capacity, Stage 2min capacity, Stage 2 max capacity, Stage 3 min capacity and Stage 3 maxcapacity.
 17. The method of claim 16, wherein the Stage 0 capacityreferences a unit capacity when the first and second fixed speedcompressors and the variable speed compressor operate in the off state;wherein the Stage 1 min capacity references the unit capacity when thevariable speed compressor is operating at Stage 1 min, where both thefirst fixed speed compressor and the second fixed speed compressor areturned off; wherein the Stage 1 max capacity references the unitcapacity when the variable speed compressor is operating at Stage 1 max,where both the first fixed speed compressor and the second fixed speedcompressor are turned off; wherein the Stage 2 min capacity referencesthe unit capacity when the variable speed compressor is operating atStage 2 min where the first fixed speed compressor is turned on; whereinthe Stage 2 max capacity references the unit capacity when the variablespeed compressor is operating at Stage 2 max, where the first fixedspeed compressor is turned on and the second fixed speed compressor isturned off; wherein the Stage 3 min capacity references the unitcapacity when the variable speed compressor is at Stage 3 min and boththe first fixed speed compressor and the second fixed speed compressorare turned on; and wherein the Stage 3 max capacity references the unitcapacity when both the first fixed speed compressor and the second fixedspeed compressor operate in the on state.
 18. The method of claim 11,wherein in (c), the PI capacity value is compared to one or morepredetermined values to determine the applicable operational stage. 19.The method of claim 18, wherein the predetermined value is based on thetype of compressors utilized.